Thin-walled plastic containers of thermoplastic material such as PET usually are produced from injection-molded parisons, so-called preforms, and molded to form containers in a two-stage stretch blow molding method. During the processing, the preforms are heated to a defined process temperature to allow for the molding process during stretch blow molding. Here, the preforms first are heated by means of stretch blow molding machines in a heating path to sufficiently heat the relatively thick-walled preforms across the entire cross-sections thereof, so that a hollow body can be formed with the ensuing molding process by means of a stretch blow molding method. Heating in the heating path may take place by means of infrared radiation, for example. Such infrared radiation has the advantage that the radiation partly penetrates the material (PET, in most cases), so that good heating of the preform across the entire cross-section thereof can be guaranteed in this way. When passing through the heating path, the preforms may, for example, be placed on a heating mandrel, so that they can be set in rotation for achieving uniform heating across the circumference when passing the IR radiators.
A preform substantially consists of a mouthpiece region already fabricated to final dimensions and a body region still to be molded. The two regions must be kept thermally separated from each other as far as possible during tempering of the preform, since when the mouthpiece is heated too strongly, the dimensional stability thereof may be compromised and/or the mouthpiece may be deformed. Yet, especially the region directly below the mouthpiece needs to be heated more strongly so as to be able to ensure a reliable blow molding process.
In known tempering devices, certain regions of the preform are heated more strongly, the neck region in particular. WO 92/15442 A1 discloses a method and a device for producing blow-molded hollow bodies from plastics, such as PET. Here, a cast, hollow preform in a heated state is placed in a blow mold and brought into a desired hollow-body shape while stretching the material of the wall thereof, wherein the wall is subjected to different degrees of stretching in from region to region, depending on the desired hollow-body shape. In addition, regions of the wall, a neck region in particular, are heated additionally prior to blow molding. In this method, transition regions of the wall of the preform between wall regions with different degrees of stretching are to be heated additionally.
DE 20 2008 005 252 U1 describes a lamp device as well as a heating path for heating preforms having a support ring, for the production of containers, such as bottles. The lamp device forms a radiation source and includes a reflector for the reflection of radiation emitted from the radiation source. The reflector extends over an angle greater than 200 degrees in circumferential direction of an axis passing through the radiation source. Hereby, directed radiation is to be generated, tempering the preform in preferred regions.
Furthermore, DE 696 23 606 T2 discloses a method and a device for selectively heating a preform of a container, such as a bottle, of thermoplastic material. Heating takes place by means of lamps generating directed radiation. After the heating process, the hot preform is subjected to a blow molding or blow draw molding process. The radiation of the lamps is emitted in such a focused manner with the aid of optical focusing means as desired onto the preform such that the same substantially is heated only in the regions to be molded.
The infrared radiation is absorbed by the preform material up to a certain degree only. This absorbance depends on the wall thickness of the preform, on the preform material used for the preform, on additives added, and on other factors, as the case may be. However, there still is radiation piercing the walls of the rotationally symmetrical preform and then again penetrating on the respective opposite side. This distribution of radiation can be utilized advantageously, because the energy input into the preform can be increased in this way. Given this distribution of radiation, however, radiation proportions having an angle of radiation directed upward to the mouthpiece are deflected directly to the mouthpiece or the support ring and heat these regions.
For targeted distribution of radiation, and particularly for shielding the threaded portion of a preform to be heated, JP 2008 000 972 A proposes introducing, into the interior of a preform to be heated from the outside by means of infrared radiation sources, a rod of titanium-containing metal at least partly reflecting a radiation proportion piercing the walls thereof.
In the known prior art, however, overheating of the support ring still occurs in some cases, so that the same becomes deformed by the power transmitted via the blowing nozzle during the blow molding process.